The present invention relates to hollow blow-molded containers of a biaxially oriented thermoplastic material, and more particularly to thin-walled plastic containers configured to accommodate partial evacuation without adverse effects on their appearance.
Lightweight, thin-walled containers made of thermoplastic materials such as polyester resin and thermoplastic polymers containing at least 50% by weight polymerized nitrile-group-containing monomer (hereinafter "nitriles"), are well known in the container industry. For example, polyethylene terephthalate (PET) has a wide range of applications in the field of containers for foodstuffs, flavoring materials, cosmetics, beverages and so on. PET can be molded, by orientation-blowing, into transparent thin-walled containers having a high stiffness, impact strength and improved hygienic qualities with a high molding accuracy. Strong, transparent and substantially heat resistant containers may be produced by the biaxial-orientation blow-molding process in which a parison is oriented both laterally and longitudinally in a temperature range suitable, for such orientation. Nitrile and heat set PET containers are particularly heat resistant. Biaxially-oriented blow-molded containers have greater stiffness and strength as well as improved gas barrier properties and transparency.
When a thermoplastic container is filled with a hot liquid (such as a liquid sterilized at a high temperature) and sealed, subsequent thermal contraction of the liquid upon cooling results in a partial evacuation of the container which tends to deform the container walls. Backflow into a filling mechanism and the use of vacuum filling equipment during filling operations can similarly create a partial vacuum inside the container, resulting in its deformation. Such deformation typically concentrates at the mechanically weaker portions of the container, resulting in an irregular and commercially unacceptable appearance. Further, if the deformation occurs in an area where the label is attached to the container, the appearance of the label may be adversely affected as a result of container deformation.
By increasing the wall thickness of the container it is possible to some extent to strengthen the container walls and thus decrease the effects of vacuum deformation. However, increasing the wall thickness results in a substantial increase in the amount of raw materials required to produce the container and a substantial decrease in production speed. The resultant increased costs are not acceptable to the container industry.
A prior attempt to reduce the effects of vacuum deformation is disclosed in U.S. Pat. No. 3,708,082 to Platte. Platte discloses a container with four flat wall panels comprising the body portion of the container. A rib circumscribes the entire container in a region below the handle and serves to rigidify the side wall portions in a circumferential direction. The rib also acts as a hinge to allow limited inward collapsing of the container along selected regions.
Another prior approach to reduction of the effects of vacuum deformation is disclosed in Japanese Application No. 54-30654. In this approach, a container is provided with a plurality of recessed panels, separated by lands, which allow uniform controlled inward deformation so that vacuum effects are accommodated in a uniform manner without adverse effects on the appearance of the container.
Prior art approaches have included the use of collapse panels (i.e., indented surface areas which provide for controlled, quantified collapse) to overcome thermal deformation of the container. However, problems have developed in containers designed with large collapse panels, i.e., panels having a width greater than 30-40 mm. While large collapse panels accommodate a greater degree of controlled deformation, as the width of the collapse panel increases, the strength of the container body decreases. Thus, bulging in the area of the collapse panels occurs, even with a partial vacuum inside the container. Furthermore, formation of heat set PET containers with large collapse panels involves serious shrinkage control problems which result in an undesirably rippled surface of the container.